The first rule about sea year is that the rules are not hard and fast. DD's sea partners have been a mix, sometimes deck sometimes engine. She liked having an engine partner because they had similar schedules and actually got to do things together. At one point, she was the 3rd engine cadet on a ship for 2 weeks. She needed about 10 more days and the ship was willing to take her.
DD's rule about choosing a sea partner, when you actually get to choose, is don't go with a good friend. The stresses of sea year have been known to ruin friendships. That said, other mids do well partnering with good friends. So your mid's mileage may vary.